Long-range RF communication - why narrowband …

1 Executive summaryA key parameter for a wireless communica-tion system is the communication range. Fundamentally, the range is dictated by the communication data rate. There are several technical options to increase range by reduc-ing data rate. Two solutions are compared here; scaling the receiver bandwidth to the signal to reduce noise seen by the receiver ( narrowband system), and adding coding gain on a higher rate signal to combat the high receiver noise in a wideband is shown that adding coding gain for long range communication will negatively affect sys-tem performance by: Greatly reducing spectrum efficiency, , reducing the number of devices that can communicate in a given area, opposite of the industry trend to increase number of radio frequency (RF) nodes Reducing coexistence properties (protec-tion against interference)

2 , which further reduces communication reliability and number of devices in a given area Decreasing battery lifetime as coding gain signals need very long leader sequences to recover the information signal from the very strong noise component narrowband communication is a proven way of achieving Long-range RF communication and offering superior availability and scalability ver-sus systems based on coding gain principles. Long-range RF communication : Why narrowband is the de facto standardIntroductionA key parameter for a wireless communication system is the communication range. In many cases the range application requirements will be the deciding factor when choosing an RF solution.

3 In this white paper, we will use the typical requirements of an RF-enabled water or gas meter to discuss technology options for Long-range RF communica-tion, but the factors are also valid for most other types of battery-operated RF nodes. Water and gas meters, often referred to as flow meters, are battery driven and typically have a battery lifetime requirement of between 10 and 20 years. The long battery lifetime puts strong restrictions on choice of network architecture. To be able to guarantee battery lifetime, these meters mostly use a star-network topology, since using these meters as repeaters/routers will make battery lifetime non-deterministic, and challenge the battery lifetime requirements. In a deployment, the network consists of many meters and few concentrators to collect metering data.

4 The concentrator is typically a more costly node than the meter, since the lower volumes make it possible to have more advanced radios ( , software-defined radio), high-current low-noise amplifiers (LNAs) (typically mains powered), better antenna technology, SAW filters, etc. To get wide coverage the concentrator must also be put in a good spot for RF. These types of basestation spots usually carry a relatively high yearly fee; hence the number of concentrators needs to be as low as possible. In a star network topology, the longer the range of the RF links, the fewer concentrators are considerations, narrowband versus coding gain solutionsFor a given output power (often defined by governmental RF regulations), the range of the RF link is determined by the data rate, , lower rate provides longer range due to increased sensitivity for the receiver.

5 There is, of course, also a trade-off, since very low rates mean very long time on air, which will in turn reduce the battery lifetime. Having very long telegrams also Terje LassenProduct Line Manager, TI Industrial Low Power RFTexas InstrumentsWHITE PAPERLong-range RF communication : Why narrowband is the de facto standard March 20142 texas Instrumentsincreases the probability of interference/collisions with other wireless systems. So in practical installations, Long-range systems typically use a reasonably low data rate, typically down to ~1 kbps, to optimize the range and transmission time balance. narrowband technique for long range and reasonably low date rate is widely accepted by the industry since it gives the optimum tradeoff between range and the transmission systems are defined as having less than 25 kHz bandwidth and provide an excellent link budget due to low in-band receive noise (narrow receive filters remove most of the noise).

6 Channel spacing with 10-kHz receive bandwidth is commonly used. Examples of such systems are police and safety radios, maritime communication systems, social alarms and the new 169-MHz wireless M-bus (wM-Bus) standard for metering applications in Europe. For wM-Bus, 169-MHz narrowband was chosen to get maxi-mum range for water and gas meters to enable fixed network deployments with very few mentioned above, the range is fundamentally determined by the data rate. An alternative to narrowband communication is to use wideband, high-data-rate communication and add coding gain. Using coding gain does not improve the sensitivity or range; it is simply another way of representing the data. Given the same net data rate/throughput, narrowband and coding gain systems will have similar range.

7 This is easiest to see by means of an key parameter of an RF system is the receive bandwidth (RXBW). The receive bandwidth is a major fac-tor in setting the noise floor of the system; PdBm = 174 + 10log10 (RXBW), , the noise floor scales with the receive bandwidth ( 174 dBm is the thermal noise floor at room temperature in a 1-Hz bandwidth). Using this formula, we can calculate noise floor for different channels:1-MHz channel: PdBm = 174 + 10log10(1 MHz) = 114 dBm100-kHz channel: PdBm = 174 + 10log10(100 kHz) = 124 dBm10-kHz channel: PdBm = 174 + 10log10(10 kHz) = 134 dBmAs seen above, a 10 increase of RXBW increases the noise floor by 10 dB. To get the same range as an uncoded narrowband system, 10 dB coding gain must be used for a 100-kHz system and 20 dB coding gain must be used for a 1-MHz system.

8 This example shows the coding gain does not offer increased sensitivity over a narrowband system; it is just a different way of representing the data. Adding more coding gain will not help, as then you need to either reduce the net data rate or increase the RXBW to fit the signal. It is important to understand this fundamental relation when doing trade-offs for Long-range RF communica-tion main drawback of using a coding gain solution is the very low spectrum efficiency. The example above shows this very clearly. Compare sending a 1-kbps signal in a 10-kHz channel using narrowband versus sending the same 1-kbps signal in a 100-kHz channel using coding gain. The waste of spectrum is quite obvious, as you send a lot of redundant data in the coding to compensate for the higher noise floor.

9 It is easy Drawbacks with coding gain solutions3 texas Instrumentsto see that in the same 100-kHz bandwidth used for coding, there is room for ten narrowband channels. Network capacity is hence a major drawback of coding gain higher receive sensitivity for less spectrum efficiency (higher bandwidth) by spread spectrum goes against regulatory requirements and worldwide industry practice for better spectrum utilization. The growth in demand for wireless connectivity has increased the demands on radio spectrum around the world. Govern-ments and regulatory bodies are placing increasing pressure to improve radio systems spectrum to kHz and even ultra-narrowbanding to kHz RF channels are both well-estab-lished solutions to increasing spectrum efficiency.

10 Narrowbanding mandate by FCC: On January 1, 2013, all public safety and business industrial land mobile radio systems operating in the 150 512 MHz radio bands must operate using minimum equivalent efficiency technology, , at least kbps throughput per mandate is the result of an FCC effort to ensure more efficient use of the spectrum and greater spectrum access, which effectively ban use of coding gain schemes for frequencies below 512 MHz in Similar trends can be seen in Europe and other regions to formally enforce spectrum coding gain is used, it is possible to have several orthogonal codes in the same channel, but then the protection between these is given by the coding gain only. In the above example, having 10-dB coding gain, will give less than 10-dB protection against another meter in the same channel.